This invention relates generally to electrochemical machining and more particularly to monitoring gap sizes and workpiece thicknesses during electrochemical machining operations.
Electrochemical machining (ECM) is a commonly used method of machining electrically conductive workpieces with one or more electrically conductive tools. During machining, a tool is located relative to the workpiece such that a gap is defined therebetween. The gap is filled with a pressurized, flowing, aqueous electrolyte such as sodium nitrate. A direct current electrical potential is established between the tool and the workpiece to cause controlled deplating of the electrically conductive workpiece. The deplating action takes place in an electrolytic cell formed by the negatively charged electrode (cathode) and the positively charged workpiece (anode) separated by the flowing electrolyte. The deplated material is removed from the gap by the flowing electrolyte, which also removes heat formed by the chemical reaction. The anodic workpiece generally assumes a contour that matches that of the cathodic tool.
For a given tooling geometry, dimensional accuracy of the workpiece is primarily determined by the gap distribution. The gap size should be maintained at a proper range. Too small a gap (such as less than 100 micrometers in a standard ECM operation) could lead to arcing or short-circuiting between the tool and the workpiece. Too large a gap could lead to non-uniform machining as well as a reduction in machining rate. Monitoring and controlling the gap size between the tool and the workpiece is thus important for ECM tolerance control. In addition, in process measurement of workpiece dimensions is important in many operations. For example, in machining a rotor blade for a gas turbine engine, the blade thickness should be directly measured during machining so that the desired blade thickness is obtained.
Lack of suitable means for sensing gap sizes and workpiece dimensions may hinder ECM accuracy control. Without such means, many rounds of costly trial-and-error experiments must be run to obtain the gap size changes that occur during the machining process. Gap size can change significantly during the machining process, partly because the conductivity of the electrolyte may change in the gap due to Joule heating or gas bubble generation on the tool surface. Variation and inaccuracy in tool feed rate and tool positioning can also contribute to changes in gap size and workpiece thickness. In process detection of gap sizes and workpiece dimensions is thus needed to improve ECM process control.
Several types of ECM sensors have been developed over the years. An ECM control method using ultrasonic sensors is described in U.S. Pat. No. 5,672,263 issued Sep. 30, 1997 to David A. Raulerson et al. This control method is used in connection with electrochemical machining of large cylindrical workpieces, where a machining head is located outside of the workpiece for machining its outer surface. One or more ultrasonic sensors are located within the cylindrical workpiece for monitoring workpiece wall thickness during the machining operation. Movement of the ultrasonic sensors relative to the workpiece results in significant signal noise and inaccuracy because of local workpiece metallurgical inhomogeneity. Furthermore, the Raulerson et al approach is limited to workpieces having a large, inner opening for containing the sensors and storing the fluid through which the ultrasonic waves propagate. By way of example, the Raulerson et al approach cannot be used while electrochemical machining small, compact workpieces, such as rotor blades used in gas turbine engines, because ultrasonic sensors cannot be placed inside such workpieces. In addition, Raulerson et al does not measure gap size. It is intended to only measure a workpiece wall thickness near a wide open space.
Recently, an approach to in-situ measurement of gap size and workpiece thickness has been proposed for ECM process control. In this approach, a single ultrasonic sensor is embedded in the ECM tool, and the gap size and workpiece thickness are obtained from ultrasonic time-of-flight measurements. The sensor generates an ultrasonic wave that propagates through the tooling, through the electrolyte in the gap and then through the workpiece. The sensor will receive reflections from the surface of the tool, the front side of the workpiece, and the back side of the workpiece. By comparing the time at which each of these reflected signals is received, the gap size and workpiece thickness can be determined.
In most situations, this approach works quite well. However, with some workpiece materials, material impedance mismatches between the tool, solution and workpiece can be extremely large. Due to such large impedance mismatches, and signal attenuation in the materials, the ultrasonic signals transmitted through the workpiece and then reflected from the back side of the workpiece can become extremely small and difficult to detect. Furthermore, due to the inhomogeneity of some workpiece materials, the acoustic velocity of these materials could vary from location to location, thereby reducing the accuracy of the thickness measurements. Accordingly, it would be desirable to have an approach for the in-situ measurement of gap size and workpiece thickness that is independent of workpiece material.